The author has come up with a new aero combined cycle innovation to overcome aero's relatively low exhaust temperatures compared to the latest HD gas turbines, whereby the latest uprated aero gas turbines can be competitive with the HD combined cycles in terms of CC efficiency and sequential installed cost, when considering the part load heat rate and 10 minute start up time of individual units.
The aeroderivative Trent gas turbine
Each application and particular site location can then be evaluated for shipment times, installation times, maintenance costs, down time, operating flexibility, time to install the plant, ramping up and down times and yearly operating profiles (monthly, daily and hourly) as to what might dominate.
This article describes how this straight forward, practical and cost effective scheme can be provided. No new equipment development is required – parts are simply reshuffled; new HRSGs do not have to be developed. This scheme could motivate aero engine manufacturers and aero power plant users to install more units for quick delivery, fast installation and step by step completion, first with the aeros and then with the HRSGs and steam turbine generators.
There seems to be a trend for companies to order blocks of 4 to 6 aero gas turbines for a given location. The idea of applying the Rice Split Stream Boiler concept is now being studied to obtain high combined cycle efficiency for aero units with relatively low exhaust temperatures compared to the HD units. Presently, the aero CCs have efficiency levels of about 52 percent compared to the HD's 60 percent. The aero CCs are also costly to install.
A batch of 5 of the uprated GE LM 6000s ("Combo 5 CC") can be installed in one power plant to produce 255 MW of flexible gas turbine power. Each gas turbine will exhaust into a three drum, mostly conventional, HRSG but without the full superheater or the reheater, to produce 1800 psia steam with 840 o F superheat temperature. One of the HRSGs, the centrally located one, will be equipped with supplementary firing in front of the boiler and will include parallel superheaters and reheaters for all 5 units.
New side-of-the-exhaust design
The throttle and reheat temperatures will both be 1100 o F, the standard for the new GE Flex HD 9FB design. The steam turbine will be designed for the 1100/1100 o F steam with proper admission points and will be the new side exhaust design. The gas temperature exiting the central supplementary fired HRSG will be, by design, the same as the LM-6000 (880 o F). Fuel gas heating will be incorporated.
Initial calculations show that for an 1800 psia 1050/1050 o F three pressure (drum) system, without fuel gas heating, the split steam flow concept, applying the new 5 unit LM 6000s, will have a CC cycle efficiency level approaching 58 percent to challenge that of the earlier HD CC unit. The total plant output will be about 375 MW, 255 MW for the gas turbines and 120 MW for the single reheat steam turbine. However, the CC efficiency level can be raised from 58 percent to 60 percent by going from 1050/1050 o F to 1100/1100 o F and by adding fuel gas heating to match the new GE FlexEfficiency HD 9FB system, but the cost will be higher in terms of $/ installed KW. The total plant output will be about 380 MW.
The standard 5 unit new LM 6000 combined cycle efficiency is calculated to be just shy of 54 percent gross (53percent, net) with the steam turbine producing 83 MW of power for a total of 338 MW to show the comparison of the two cycles. In the second part of this series, the author discusses the different types of heat losses in combined cycles and talks about ways and means of addressing this loss and increasing efficiency.
(This article is the first part of a series by the author.)
Ivan G. Rice was past chairman of the South Texas Section of ASME (1974 - 75), past chairman of the ASME Gas Turbine Division (now IGTI) (1975 - 76). A Life Fellow Member of ASME and Life Member of NSPE/TSPE, he has authored many articles and ASME papers on gas turbines, inter-cooling, reheat, HRSGs, steam cooling and steam injection.